The purification of a liquid by reducing the concentration of ions or molecules in the liquid has been an area of substantial technological interest. Many techniques have been used to purify and isolate liquids or to obtain concentrated pools of specific ions or molecules from liquid mixtures. Known processes include electrodialysis, liquid chromotography, membrane filtration, reverse osmosis, ion exchange and electrodeionization. As used herein, the term electrodeionization refers to the process wherein an ion exchange material such as an ion exchange resin is positioned between anionic and cationic diaphragms or membranes. In contrast, the term electrodialysis relates to a process which does not utilize ion exchange materials positioned between the anionic and cationic diaphragms.
Early suggestions for treating liquids using processes that ultimately evolved to electrodeionization in the modern sense of the term were described by Kollsman in U.S. Pat. Nos. 2,689,826 and 2,815,320. The first of these patents describes an electrodialysis apparatus and process for the removal of ions from a process stream. The ions are removed from the stream as it flows through a depleting chamber defined, in part, by anion and cation permeable diaphragms. The ions pass through their respective permeable membranes and into a second volume of liquid in a concentrating chamber under the influence of an electrical potential which causes preselected ions to travel in a predetermined direction. The volume of the liquid being treated is depleted of ions while the volume of the second liquid is enriched with the transferred ions and carries them in concentrated form.
The second of these patents describes the use of macroporous beads formed of ion exchange resins as a filler material between the anionic and cationic diaphragms. The ion exchange resin acts as a path for ion transfer and also serves as an increased conductivity bridge between the membranes for movement of ions. These patents represent the primary structural framework and theory of electrodeionization as a technique.
Significantly improved electrodeionization systems have more recently been disclosed in U.S. Pat. No. 4,925,541 to Giuffrida et al. and U.S. Pat. No. 4,931,160 to Giuffrida, the teachings of which in their entireties are incorporated herein by reference.
In these references, the depleting and concentrating chambers are present as cell pairs each having a rectangular configuration that is 16.3 inches wide and 33.75 inches long. Each cell pair consists of a diluting compartment, in which contaminants are removed from a first process stream (the diluting stream), and a concentrating compartment in which the contaminant ions are concentrated in a second process stream (the concentrating stream). For reference purposes, the process stream in the diluting compartment is referred to as a dilute or diluting stream, and the process stream in the concentrating compartment is referred to as the concentrate or concentrating stream.
The diluting compartment is defined by a spacer molded from polypropylene and having dimensions the same as that of the rectangular cell configuration and a thickness of 0.090 inches. The compartment is filled with ion exchange resin, preferably in the form of beads. Electrodialysis membranes of the same rectangular dimensions as the spacer are bonded to both sides of the spacer to form the top and bottom walls of the diluting compartment. The membranes differ in that one is an anion exchange membrane, whereas the other is a cation exchange membrane. A series of thin parallel channels, each 1.08 inches wide, 25.75 inches long, and 0.090 inches deep define flow paths for the process stream which is flowed through the diluting compartment, (the diluting stream). The diluting stream is flowed into the diluting compartment and is directed into the channels through a manifold. The manifold includes numerous obstructions which are designed to prevent channeling and to evenly distribute the flow among the channels. The spacer includes a flow inlet and flow outlet located on diagonally opposite corners to equalize pressure drop across all flow paths from the inlet to the outlet.
The concentrating compartment is also defined by a spacer having the same rectangular dimensions as that of the diluting compartment. The spacer of the concentrating compartment may be thinner than that of the diluting compartment, however, as will be described shortly, in some applications the concentrating spacer is of the same thickness as the diluting spacer. In general, the concentrating compartment contains an inert screen which serves to mix the concentrating stream flowing therethrough. Alternatively, if the apparatus is to be operated in a polarity reversal mode, or in an all-filled non-polarity-reversal mode, the screen is replaced with an ion exchange resin or ion exchange resin mixture.
The ion exchange resins and membranes may be those described in the aforementioned Giuffrida patents. Alternatively, the resins and membranes may be those described in co-pending U.S. patent application Ser. No. 07/628,338, filed Dec. 17, 1990, and its co-pending continuation-in-part, U.S. patent application Ser. No. 07/841,021 filed Feb. 25, 1992, the teachings of each of which are incorporated by reference herein.
During assembly of an electrodeionization unit, the cell pairs are stacked and compressed between molded polypropylene end blocks backed by aluminum end plates. The end plates are attached to each other by tie-bars located along the periphery of the end plates. The end blocks, end plates and tie-bars are referred to as the closing mechanism.
Although the electrodeionization apparatus described above has found wide commercial application, some shortcomings of the current cell design exist. For example, the active area of the membranes on both sides of the dilute spacer is the area which acts as the boundary of each of the flow channels. Typically, the total active area for membrane is 278 square inches which is only approximately 50% of the total membrane area. The other 50% of the membrane area is unusable because it is in contact with impermeable areas of the spacer or is sealed by gaskets or adhesives. As the overall efficiency of an electrodeionization apparatus depends, in part, on the total membrane area available for ion exchange, it is clear that ion exchange performance can be enhanced by minimizing the amount of unusable membrane surface area. Flow distribution through the channels is also a concern, particularly as the flow rate through a cell pair is varied from minimal to maximal flow.
The flow capacity of an individual electrodeionization unit can be expanded only by adding cell pairs, which necessitates disassembly of the closing mechanism, insertion of cell pairs and installation of longer tie-bars.
Finally, the perimeter of the cell pairs must be sealed from the environment. This is achieved using adhesives that are typically elastomers. The current rectangular electrodeionization modules have, on occasion, been subject to leakage which is undesirable, particularly in small electrodeionization units housed inside enclosures.
Thus, a need exists for an electrodeionization apparatus which allows a greater utilization of the ion exchange membrane.
A need also exists for an electrodeionization apparatus having a simpler, and more secure means for maintaining a seal between the respective diluting and concentrating compartments.
An additional need exists for an electrodeionization apparatus having a modular design.
A further need exists for an electrodeionization apparatus that can be fabricated using a higher level of automation.